node - The node in the parse tree representing the element being parsed. It is important
to note that at the time this method is called the node contains no child element nodes,
only child attribute nodes.

context - The container to be used as context for child binding.

parse

Even though the spec allows an instance of ParamterValueType to have multiple expressions as
children, it is more often that there is only one. Therefore this binding returns the first
expression it finds or null. If a subtype needs multiple expressions they should ovveride.